1. Field of the Invention
The present invention relates to thermal cracking of hydrocarbons and more particularly, to a process for selectively producing petrochemical products such as olefins and aromatic hydrocarbons (hereinafter referred to simply as BTX) by thermal cracking of hydrocarbons.
2. Background of the Invention
The tube-type thermal cracking process called steam cracking is well known and has been heretofore used to convert, into olefins, light gaseous hydrocarbons such as ethane and propane as well as liquid hydrocarbons such as naphtha and kerosine. According to this process, heat is supplied from outside through tube walls, thus placing limits on the heat transmission speed and the reaction temperature. Ordinary conditions adopted for the process include a temperature below 850.degree. C. and a residence time of from 0.1 to 0.5 seconds.
Another process has been proposed in which a tube of a smaller diameter is used to increase the cracking severity and to effect the cracking within a shorter residence time. However, this process is disadvantageous in that because of the small inner diameter, the effective inner diameter is reduced within a short time due to the coking on inner walls. As a consequence, the partial pressure of hydrocarbons increases with an increasing pressure loss in the reactor, causing worse selectivity to ethylene. To avoid this, it is necessary to shorten decoking intervals. This will cause the great disadvantage that because of the reduction of working efficiency of a cracking furnace and also of an increasing number of heat cycles accompanied by the decoking operation, the cracking apparatus is likely to be damaged. In view of the limitation on the apparatus and reaction conditions, starting materials to be cracked are restricted, at most, to gas oils. In other words, the process cannot be applied to heavy hydrocarbons such as residual oils. This is because high temperatures and long reaction times inevitably involve side reactions of polycondensation, so that coking takes place violently and a desired gasification rate cannot be attained, leading to low yields of useful components. It will be noted that the term "gasification rate" used herein is intended to mean a ratio by weight, to an amount of a starting hydrocarbon, of an amount obtained by subtracting C.sub.5 and heavier hydrocarbons except for BTX from a hydrocarbon fed to a reaction zone.
Once a starting material is selected, specific cracking conditions and a specific type of apparatus are needed in conformity with the requirements for the single starting material and intended final products. Thus, difficulties are encountered in poor selectivity to starting materials and products with little versatility. For instance, a currently used, typical tube-type cracker is chiefly used for the production of ethylene, so that it is difficult to arbitrarily vary yields of other fundamental chemical products, produced simultaneously, such as propylene, C.sub.4 fractions and BTX according to a supply and demand balance. From this, it will be seen that since selectivity to ethylene, which is obtained in high yield by cracking of other substitute starting materials, e.g. heavy hydrocarbons, at high severity, is secured by naphtha, the great possibility of naphtha which is convertible into propylene, C.sub.4 fractions such as butadiene and a BTX product is sacrificed. If the yield of ethylene is increased, propylene and C.sub.4 fractions irreversibly decrease. This is inevitable in thermal cracking reactions.
Several processes have been proposed for relaxing the limitations placed on both starting materials and final products as discussed above.
In one such process, a liquid hydrocarbon such as crude oil is used as a fuel to generate a hot gas, with which hydrocarbons are thermally cracked under conditions of a pressure of 5 to 70 bars, a reaction temperature of 1315.degree. to 1375.degree. C. and a residence of 3 to 10 milliseconds. In this process, an inert gas such as CO.sub.2 or N.sub.2 is passed in the form of a film in the direction from the burner for the hot gas toward the reaction zone, whereby coking is suppressed, making it possible to crack heavy oils such as residual oils.
Another process comprises the steps of partially burning hydrogen to give hot hydrogen gas, and thermally cracking various hydrocarbons including heavy oils in an atmosphere of the hot hydrogen gas under conditions of a reaction temperature of 800.degree. to 1800.degree. C., a residence time of 1 to 10 milliseconds, and a pressure of 7 to 70 bars, thereby obtaining olefins. The thermal cracking in an atmosphere having an excess of hydrogen ensures rapid heating, super-short-residence-time cracking, and suppression of coking. In addition, even heavy oils can be cracked. However, this process is disadvantageous in that recycle and separation power for hydrogen, and hydrogen makeup and pre-heating energy impose a considerable economical burden on the process.
These prior art processes need very severe reaction conditions to obtain olefins in high yield from heavy hydrocarbons. As a result, olefins obtained as a final product are mainly composed of C.sub.2 fractions such as ethylene and acetylene, with the problem that it is difficult to operate the process so that propylene, C.sub.4 fractions and BTX are simultaneously produced in high yields.
The present inventors have made intensive studies to develop thermal cracking processes for producing olefins and particularly C.sub.3 and C.sub.4 olefins such as propylene and butadiene from a wide variety of hydrocarbons including light to heavy hydrocarbons. As a result, it was found that when methane was fed to a reaction atmosphere, the yields of olefins increased remarkably and selectivities to C.sub.3 and C.sub.4 olefins also increased. The findings were already proposed in Japanese Patent Application Nos. 57-38684, 58-25797, 58-34928 and 58-41932.
The present inventors have made further studies on the mechanism of methane in the thermal cracking process to significantly improve the yields of products and selectivities to individual products by proper control of the reaction atmosphere for thermal cracking. As a result, we found that when methanol was added to the reaction atmosphere, olefins and BTX could be produced in high yields and high selectivities. The present invention is accomplished based on the above finding.